Many medical devices incorporate elongate shafts such as tubes which are intended for insertion into and through passageways of a living body such as those of the urethral tract and the cardiovascular system. The most common type of this general grouping of medical devices are known as catheters. Exemplary catheters include those designated for urological, angioplasty and valvuloplasty uses, that is, adapted respectively for insertion into the urethra, the lumen of a blood vessel and heart passageway of a living body, normally a human body.
Because of the intended use of such medical devices certain parameters need to be satisfied by the material from which the elongate shaft is manufactured. The material must fulfill such requirements as softness, good kink resistance, good dimensional stability, processability, for example ease to form and glue, and the possibility to be sterilized by radiation, steam, ethylene oxide or other means. There is further the need for the material to accept a surface treatment which will impart desired surface properties to the medical device, such as hydrophilicity. To this latter end, it is of utmost importance to find a substrate material that enables the possibility to coat the substrate.
Further, a well-recognized problem with hydrophilic coatings or layers has been that the hydrophilic polymer surface may lose water and dry out when it comes in contact with e.g. a mucous membrane, such as when the catheter is inserted into the urethra. This occurs because of a difference between the osmotic potential of the hydrophilic surface and the osmotic potential of the mucous membrane. The mucous membrane has a higher osmotic potential, i.e. a higher salt concentration, than the hydrophilic surface. This difference in osmotic potential causes the water to go from the hydrophilic surface layer to the mucous membrane so that the difference in the salt concentration will be counter-balanced. Naturally, this affects the low-friction properties of the hydrophilic outer surface coating, and may lead to pain and injuries of the patient. For this reason, the present applicant has previously developed an improved hydrophilic coating, in which an osmolality-increasing compound was applied to a non-reactive hydrophilic polymer surface, thereby producing a more stable hydrophilic surface, as is disclosed in EP 217 771. Hereby, the theretofore prevailing problem of the hydrophilic coating drying out when inserted into the urethra, thus rendering the article insufficiently hydrophilic, was alleviated.
Similar hydrophilic coatings incorporating an osmolality-increasing compound are discussed in WO 94/16747 disclosing a process in which the osmolality-increasing compound is added during the process of applying the hydrophilic coating to the base material, EP 586 324 and EP 591 091 disclosing a hydrophilic coating comprising a non-dissolved, solid osmolality-increasing compound e.g. in the form of a powder or grain, and EP 991 702 disclosing a cross-linked hydrophilic coating comprising a water soluble osmolality-increasing compound.
However, these known methods and coatings are affected by some problems. For example, the production processes, involving different manners of incorporating the osmolality-increasing compounds in the coatings, are rather tedious cumbersome and costly. Further, the properties of the resulting, wetted hydrophilic surface coating to be inserted into the patient are, at least to a certain extent, affected by parameters of the wetting process, such as the quantity of wetting fluid used for the wetting, the constituents of the chosen wetting fluid, and the time period during which the wetting is carried through. Since several such parameters may be unknown beforehand, and may vary to a significant degree, the properties of the resulting, activated coating become unpredictable as well.
Thus, there is a general problem of known medical devices with hydrophilic coatings that water retention in the coating is too low, especially after leaching, or that the coating has too poor adherence to the substrate, and/or that the means used for prolonging the water retention time and the adherence of the coating is too costly and/or harmful to the environment.
There is therefore a need for an improved substrate and or coating method for providing medical devices with a hydrophilic surface coating, which is environmentally acceptable and cost effective, and which ensures that the hydrophilic coating can be adequately adhered and is efficient in use.